I am surprised that your 5600 GS derailleur has trouble with a 28 tooth large cog. A one tooth deviation from Shimano's stated capacity should not make a huge difference in shifting performance. What sort of crank setup do you have? That said, a 5701GS rear derailleur would work with your shifters. Another possible solution would be a "9 speed" Shimano mountain bike derailleur. That is what I have on my touring bike. It give me the option to be able to use a much larger cassette than the 11-28 I currently have